The unique properties of metallic nanoparticles make them candidates for various applications where metal flake is currently being utilized. Metal flakes establish a conductive pathway by incidental contact, a relatively inefficient process because the points of contact are inherently highly resistive, essentially acting as an impurity in the system.
Electromagnetic interference (“EMI”) and radio frequency interference (“RFI”) shielding are two commercial applications that incorporate the use of highly conductive shielding materials. In such applications, shielding material is typically sprayed onto the inside of the plastic housing of devices such as mobile phones.
Presently, metal flake is currently used as the conductive constituent in shielding materials. In some methods, metal flake is formulated with adhesion promoters and other additives in an organic solvent. Some commercially available metal flake formulations include organic solvents.
When metal flake is processed—typically under comparatively high-temperature conditions—the flakes coalesce into a rough conductive network that may be used for shielding. Metal flake networks, however, are not perfectly continuous.
Furthermore, metal flake can not be sintered to form a metallic network shield, and the comparatively high temperature conditions required to form metal flake network shields limits the range of substrates on which such shields can be formed. In addition, organic solvent metal flake systems are not optimal for some applications because of the difficulties inherent in handling of the organic solvents used in metal flake formulations.
Moreover, formation of metal flake shield structures can require several minutes of processing time.
Accordingly, there is a need for a method for forming continuous, highly-conductive metallic structures capable of shielding electronic equipment from EMI and RFI, where such method allows rapid formation of such structures under moderate processing conditions, such as sintering, while minimizing the use of metal and of solvent. There is also a related need for a composition capable of forming such structures under such moderate processing conditions so as to enable formation of shielding structures on substrates unable to tolerate the harsh processing conditions associated with metal flake shielding systems.